It is well known to deposit coatings of material on a conductive core by electrophoretic deposition (EPD). EPD is a combination of electrophoresis and deposition. Electrophoresis is the movement of charged particles in an electric field. Deposition is the coagulation of particles into a mass. Applicant's own PCT application no. PCT/CA01/00634 relates generally to the production of hollow ceramic membranes by EPD, and in particular to the production of hollow, tubular ceramic electrodes by EPD for solid oxide fuel cells (SOFC).
In general, a SOFC comprises two electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of about 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:
Anode reaction:H2+O═→H2O+2e−CO+O═→CO2+2e−CH4+4O═→2H2O+CO2+8e−
Cathode reaction:O2+4e−→2O═
Known SOFC designs include planar and tubular fuel cells. Current SOFC fuel cell stack designs typically stack the fuel cells side-by-side. For example, a tubular stack design as published by Siemens Westinghouse Power Generation features tubular fuel cells arranged in a side-by-side rectangular array. The large size of the Siemens Westinghouse fuel cells (typically >5 mm diameter) and the relatively low power density (power output per unit volume) of the stack design makes such a fuel cell stack impractical for small scale applications such as portable electronic devices.
It is therefore desirable to provide a compact SOFC stack design that, in particular, can be made small enough with sufficient energy density for small scale applications.